Cisco 400-007 Exam With Confidence Using Practice Dumps

Route Distinguishers (RDs) are fundamental to MPLS Layer 3 VPN architecture (RFC 4364). They allow the service provider to distinguish identical IP prefixes from different VPN customers by adding a unique identifier to each VPN route, preventing conflicts from overlapping IP spaces.

Unique RDs ensure that the VPNv4 address family maintains separation of identical customer prefixes.

Why other options are incorrect:

B: Route Targets control route import/export policy but do not differentiate overlapping prefixes.

C: NAT is not required inside MPLS VPN for overlapping IPs.

D: VRF IDs are local identifiers on routers; not relevant for prefix uniqueness in control plane.

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The goal is to prevent AS 111 from being used as a transit AS.

The most effective method is to use an AS path access list to filter outbound BGP advertisements, allowing only prefixes originated from AS 111 (AS path contains only AS 111).

This ensures AS 111 only advertises its own prefixes and does not forward third-party prefixes between providers.

Why other options are incorrect:

A: AS-set is used for aggregation, not for transit prevention.

B: Local preference controls outbound path selection, not advertisement or transit.

D: Prefix list on inbound filters received prefixes, but does not prevent transit.

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D (Group Encrypted Transport VPN - GET VPN):GET VPN encrypts traffic across private WANs like MPLS while maintaining full-mesh routing visibility and offering centralized key management with minimal encapsulation overhead.

Other options explained:

A: S-VTI is used for point-to-point IPsec tunnels.

B: DMVPN is used for dynamic multipoint Internet-based VPNs.

C: MGRE is a tunneling protocol but lacks security features.